A fuel cell is provided in a stack structure in which fuel cell-constituting unit cells each serving as a power generation unit are stacked in a plurality of layers. Each of the unit cells has a membrane electrode assembly sandwiched by opposing separators. In recent years, there has been proposed, e.g. in Japanese Laid-open Patent Publication 2013-54872, a technique of forming recessed grooves as a fuel-gas gas flow path and recessed grooves as a cooling water flow path on front-and-back surfaces of the separator by a plurality of pit-and-bump stripes made by press molding.
According to the separator proposed in the above-cited patent reference, it is possible to achieve simplification of the structure, size reduction in the stacking direction of the fuel-cell unit cells, and moreover stacking of large numbers of unit cells. A fuel cell is mounted on, for example, a vehicle or the like while the stacked unit cells are tightened with force applied in their stacking direction, the tightening force is always applied to the unit cells of the fuel cell. Since the way how the tightening force is applied to various sites on each unit cell such as its power generation region and vicinities thereof is not necessarily uniform, the following failures as described below can be occurred due to the tightening force being always applied. Since an outer edge portion of a fuel cell separator extends outward from a separator central region opposed to the power generation region of a membrane electrode assembly, a peripheral edge of the membrane electrode assembly and a seal member for sealing the peripheral edge are overlapped at the outer edge portion of the fuel cell separator. Such interposition of the seal member tends to increase the thickness of the peripheral edge of the membrane electrode assembly, which is an object to be sandwiched with another separator, in the outer edge portion of the fuel cell separator. In contrast, in the central region of the separator, since its opposing region is the power generation region of the membrane electrode assembly and thus no other members like the seal member exists, the thickness of the membrane electrode assembly, which is an object to be sandwiched with another separator, is generally uniform. If the issues for recessed grooves as the flow path positioned and extending on the outer edge portion side and recessed grooves as the flow path positioned and extending in the separator central region are not addressed, surface pressure becomes higher around the recessed grooves extending on the outer edge portion side on account of the above-described thickness conditions, so buckling of the membrane electrode assembly or damage to the electrolyte membrane might be incurred. In the above-cited patent reference, unfortunately, coping for such failures could not be said enough, leading to an indication that there is room for improvement based on the formation of recessed grooves on the front-and-back surfaces of the separator. In addition to this, there are also needs for simple-and-easy measures for failures that can occur due to effects of the tightening force to the individual unit cells of the fuel cell as well as for enabling a reduction of the fuel cell manufacturing cost and the like.
In order to achieve at least part of the above-described problems, the present invention may be implemented in the following aspects.